WebWare recently did a good overview of the three available (sort of) Mashup editors.

After Yahoo released Pipes, which allowed people with little programming experience create web mashup applications, Microsoft and Google had to follow suit... with Popfly and Google Mashup Editor respectively.

Neither are ready for prime time, as of yet. You need an invite, and they are highly stingy with them. However, early reviews lean towards Popfly... which I believe may be the first killer app built with Silverlight.

I personally believe that enterprise mashups will be huge business in the near term... not just for cool web widgets, but also for enterprise apps. Naturally, then need to be backed with SOA and some kind of distributed single sign on system, but those are good ideas regardless if enterprise mashups take the world by storm...